In the prior art, electrical power generation systems comprising a motor, generator, rectifier and inverter are well known. Some such systems are Combined Heat and Power (“CHP”) systems used for the simultaneous production of both electricity and heat on a continuous basis. CHP systems are also known as Cogeneration or Distributed Generation systems. In a standard, non-CHP power plant, the heat generated is rejected to the atmosphere. This is not only wasteful, but also detrimental to the environment both in terms of thermal pollution and because the thermal energy which could have been used would have to be generated via other means, generally causing additional pollution. In a CHP system, the heat generated is captured and used, leading to high overall fuel utilization as well as reduced damage to the environment.
Generally, it is advantageous to install a CHP system near the point of use of the heat generated since thermal energy, unlike electrical energy, is difficult to transport. This argues for small CHP systems which can be placed in the immediate vicinity of any variety of small commercial or institutional heat loads.
At present, CHP installations in the sub-megawatt size are dominated by power generation systems utilizing natural-gas reciprocating engines. Their popularity is not due to any singular factor, but rather due to their overall value when all factors are weighed. The factors are operating efficiency, first-cost, attainable exhaust emissions, service infrastructure, durability, heat recovery, etc. Newer technologies such as fuel cells and turbines are making important inroads into the field and show promise, but progress is slow due to various issues involving the “front-end” of these systems. Neither fuel cells nor micro-turbines with re-generators are currently affordable or practical alternatives to engine generators for CHP applications. Both fuel cells and micro-turbines have unacceptably high initial cost and must be heavily subsidized by their manufacturers or by others in order to gain initial entry to the marketplace. Engine generators, especially those driven by automotive-derivative engines, by contrast, achieve low cost, even in small quantities, because they benefit from the enormous economies of scale derived from mass production of such engines.
Manufacturers typically utilize one of two generator design options with regard to engine driven, AC generator CHP systems. The first is a “synchronous” type generator, the conventional alternator technology that is used worldwide for standby and prime-power applications. The second choice is the “induction” type generator which is more or less an induction motor pushed above its synchronous rpm to export electric energy to a live utility bus. Although both generator types are well-established reliable technologies, neither is entirely satisfactory in CHP system applications for the reasons described below.
The principle advantages of a synchronous generator are standalone capability (i.e., ability to power the facility during a blackout) and non-reliance on the utility for magnetizing current or reactive power. Standalone or “black start” operation is increasingly demanded by customers, given recent security concerns for the central power grid and also the well-publicized blackouts in the US and in Europe. Regarding the reactive power issue, while it does not impact system efficiency per se, except in line losses, the positive attributes of the synchronous machine are a definite plus. However, and despite these advantages, the synchronous generator is almost never applied to small CHP systems because of the following problems which make small, synchronous packages impractical.
1. Extreme resistance by electrical power utilities to grant interconnection approval without expensive safety relay systems. Power utility engineers fear a CHP system may unintentionally electrify a portion of the grid during a power outage. This is a hazard for the utility equipment and its workers. Some power utility engineers require two completely redundant safety systems commensurate with substation design practice to protect against this hazard. This is an impossible economic burden for a small CHP system.
2. In its most common application, a small synchronous generator is used as a standby power source. The controls in these applications are simple and reliable. When used in CHP applications, on the other hand, the control system complexity changes dramatically. The CHP system must be set up to operate both parallel to the utility grid or on a standalone basis. The parallel operation involves a complex arrangement of synchronizers, reactive power controls, dual-gain governors (one for each mode of operation) and safeties to avoid dangers to linemen caused by inadvertent islanding (continued operation of the generator while connected to an isolated section of the utility grid), plus safeties to avoid synchronizing errors. However, the most general case will involve multiple CHP systems operating parallel, per a modular design philosophy, in which case the control system design becomes even more complex and factors such as lead/lag, load sharing, and reactive power sharing must be considered. This is also an impossible economic burden for a small CHP system.
As a consequence of the above described problems with synchronous generators, the small reciprocating engine CHP system market is dominated by induction-based generators. Induction generators require no paralleling equipment and modular units can be installed in multiples with no inter-unit control requirement—a truly modular design on the electrical interface side. However, despite these advantages, induction generators have the following problems
1. The inherent safety and simplicity of induction generators results in their being inoperable during a power outage.
2. Induction generators require a substantial amount of reactive power which must be obtained from the power utility grid and some utility companies penalize customers in their rate structure for power factor when a CHP system is present.
3. There is an increasing resistance by electrical power utility companies to approve CHP system interconnect applications and there are no standards committees working toward this end. It is possible that existing CHP systems will be decertified.
Thus, there is a need for a new type of CHP system that solves the above described public utility interface and other problems of prior art CHP systems.